1. Field of the Invention
This invention relates to an automatic focusing device which is suitable for a video apparatus, such as a video camera, an electronic still-picture camera or the like.
2. Description of the Related Art
Recently, video apparatuses, such as video cameras, electronic still-picture cameras and the like, have developed remarkably. In particular, in order to improve the functions and operability of such an apparatus, functions, such as an automatic focusing (AF) device and the like, are usually provided in the apparatus.
For the automatic focusing device, a method has become adoped in which sharpness of a picture frame is detected from a picture signal obtained by performing photoelectric conversion of an image of an object using an image pickup device or the like, and a focusing operation is performed by controlling the position of a focusing lens so as to maximize the sharpness of the picture frame.
In general, the sharpness is evaluated, for example, from the intensity of the high-frequency component of the picture signal extracted by a band-pass filter (BPF), or the detected intensity of the width of a defocused portion of the picture signal (the width of an edge portion of the image of the object) extracted by a differentiating circuit or the like.
When an image of a normal object is photographed, the level of the high-frequency component is small and the width of a defocused portion increases in a defocused state. As the image approaches a focused state, the level of the high-frequency component increases and the width of the defocused portion decreases. In a completely focused state, the level of the high-frequency component has the maximum value, and the width of the defocused portion has the minimum value. Accordingly, the focusing lens is controlled so as to be driven at a highest speed in the direction of increasing the sharpness if the sharpness is low, be driven at a reduced speed as the sharpness increases, and be stopped when sharpness has been obtained with high accuracy.
Such an AF control method is called a "mountaineering automatic focusing method" (hereinafter termed a "mountaineering AF"). This method is described in detail in "Automatic focusing of a television camera by means of a mountaineering servo method", NHK Giho (technical bulletin) volume 17, number 1 (Ser. No. 86).
However, in the above-described automatic focusing device utilizing a picture signal, the automatic focusing capability of the device greatly depends on an object. In particular, speed control for the focusing lens near a focus point is difficult because if the driving speed of the focusing lens is reduced, much time is needed until the device reaches the focus point, thereby reducing the speed of the AF operation. If the driving speed of the focusing lens is too large, the device passes through the focus point and therefore cannot be stopped at the focus point, causing a possibility of producing hunting, i.e., oscillation back and forth along the optical axis, near the focus point. Accordingly, it has been desired to provide an AF system which can always perform a stable driving control operation of a focusing lens.